THERMOGRAPHIC RECORDING FILMS

Abstract: There is described, in thermograhic recording films, the use of a compound containing at least two epoxide moieties in the protective layer and/or in a layer on top of the protective layer of thermographic recording films to reduce gouging and streaking of the printed image film and to reduce head build-up on the thermal printhead.

Description: THERMOGRAPHIC RECORDING FILMS CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of prior copending application serial no. 08/009,829, filed January 27, 1993. BACKGROUND OF THE INVENTION (1) Field of the Invention The present invention relates to thermographic recording films, and more specifically, it relates to the use of a crosslinking compound containing at least two epoxide moieties in a protective layer and/or in a layer on top of the protective layer of certain thermographic recording films which are to be imaged with a thermal printhead. The crosslinking compound helps to prevent gouging, to reduce head build-up on the thermal printhead, enhance print performance and to improve the image quality of the printed image. (2) Description of the Related Art There are disclosed in the art a number of image-forming systems for use in thermographic recording films. One of these image-forming systems utilizes color-forming di- and triarylmethane compounds possessing certain S-containing ring closing moieties, namely a thiolactone, dithiolactone or thioether ring closing moiety as are disclosed in European Patent No. 250,558 and U.S. Patent No. 5,196,297 of E.J. Dombrowski, Jr. et al. These dye precursors undergo coloration by contacting with a Lewis acid material, preferably a metal ion of a heavy metal, particularly silver, capable of opening the S-containing ring moiety to form a colored metal complex. As disclosed in the above-cited patents, the ability of these dye precursors to form a colored dye almost instantaneously when contacted with Ag+ renders them eminently suitable for use as color formers in thermal imaging systems employing organic silver salts, such as silver behenate. These thermographic recording films preferably include a heat-fusible organic acid material. U.S. Patent No. 4,904,572 of E.J. Dombrowski, Jr. et al, issued February 27, 1990, discloses 3,5- dihydroxybenzoic acid as a preferred heat-fusible organic acid. The above described thermal color-forming system preferably employs a thermoplastic binder, e.g. polyvinylbutyral. When imagewise heating is accomplished by means of a thermal printhead, the thermoplastic binder is in direct contact with the thermal printhead during imaging. Since thermoplastic binders soften upon the application of heat, they tend to stick to the thermal printhead during imaging. This "sticking '' interferes with the printing, adversely affects image quality, and can cause damage to the printhead. A number of ways to prevent sticking between a binder and a thermal printhead during printing have been suggested for various thermographic recording films. Many of these employ a protective or anti-stick topcoat comprising silica over the thermographic color-forming layer. These topcoats contact the thermal printhead during imaging to prevent "sticking". Another way to prevent sticking has been to employ a surface active agent to add anti-stick properties. However, these silica containing topcoats and surface-active agents have drawbacks and/or do not perform adequately when the binder employed in the coloring system is polyvinylbutyral and the support used for the thermosensitive recording film is a transparent support. For example, low surface energy materials such as silicone polymers exhibit good anti-stick properties. However, the useful silicone polymers are relatively low molecular weight silicone polymers which have a tendency to be migratory and thus cause problems, e.g. , they transfer to the back of the film if it is rolled for storage or to the back of the adjacent film if stored in sheets. In addition, because these silicones are polymers, their properties change with changes in moisture and temperature and therefore, their performance is not consistent under all conditions. U.S. Patent No. 4,583,103 issued April 15, 1986 and U.S. Patent No. 4,820,682 issued April 11, 1989 disclose protective topcoats for heat-sensitive recording papers containing a binder comprising silicon modified polyvinylalcohol and colloidal silica and/or amorphous silica. The above patents also disclose topcoats wherein said colloidal silica contains silica grains having an average particle size of from about 10 millimicrons (mμ) to 100 mμ (1 mμ = 1 nanometer (nm) ) and the amorphous silica has primary grain size of about 10 micrometers (μm) to 30 μm (1 μm = 10 3 nm) . These topcoats are disclosed as providing good printing densities, resistance to various chemicals, oils and water, and anti-sticking and anti-blocking properties. In addition, the latter patent discloses the topcoat as exhibiting excellent transparency and describes it for use on a transparent base. However, the lowest level of haze reported is 16%, a level which is higher than desirable for overhead transparency (OHT) applications. Published UK Patent Application No. 2,210,702 having a publication date of June 14, 1989 and assigned to the same assignee as the latter two patents, discloses a heat-sensitive recording material which, when it employs a topcoat as described above, e.g., silicon modified polyvinylalcohol and colloidal silica, reports a level of haze as low as 8%. However, when polyvinylbutyral is used as the binder for the color-forming materials of this invention, and a topcoat as described above, i.e. silicon modified polyvinylalcohol and colloidal silica, is employed to prevent sticking, there is poor adhesion between the topcoat and underlying polyvinylbutyral layer, as well as poor scratch resistance of the resulting film. In addition, the silicon modified polyvinyl alcohol binder is water soluble and can be rubbed off with water. U.S. Patent No. 4,985,394 issued January 15, 1991 discloses a topcoat for a thermosensitive recording material which comprises at least one inorganic pigment selected from the group consisting of silica and calcium carbonate, each having an average particle diameter of 0.1 μm or less, and a water-soluble binder, formed on the thermosensitive coloring layer. Many of these topcoats have problems of inadequate transparency and/or adhesion when coated over the polyvinylbutyral color-forming layer of the present invention. U.S. Patent No. 5,198,406 of J.M. Mack and K. Sun, assigned to the assignee of the present application, discloses a topcoat for transparent thermographic recording films using the above color- forming system. Specifically, the transparent thermographic recording films described therein comprise a transparent support carrying: (a) a dye image-forming system comprising a di- or triarylmethane thiolactone dye precursor, an organic silver salt, a heat-fusible organic acidic material, and polyvinylbutyral as the binder; and, (b) a protective topcoat layer positioned above said dye image-forming system and comprising a water-insoluble polymeric binder, a mixture of at least two colloidal silicas having different average particle diameters in the proportion, by weight, of 1 part of silica having an average diameter of 50 nm or smaller and 0.3 to 1 part of silica particles having an average diameter no more than 40% of the larger sized silica particles, the ratio of total silica to binder being at least 3 parts per weight silica to 1 part per weight binder. While the above described topcoat prevents sticking of the polyvinylbutyral color-forming layer(s) to the thermal printhead during printing, with certain high energy thermal printers, e.g. Model BX 500 high density printer, commercially available from Seikosha America, Inc., Mahwah, New Jersey and Model TDU 850 commercially available from Raytheon Company, Submarine Signal Division, Portsmouth, Rhode Island, there are the problems of gouging on the surface of the recording film and head build-up on the thermal printer. "Gouging" results in actual depressions or indentations in the recording film which can be either continuous or intermittent. Gouging is believed to be caused by high temperatures, pressure and/or sticking. "Head build-up" is the build-up of components of the thermographic recording film on the thermal printhead. Head build-up can cause streaking in the printed image, decreased image density with continued printing and damage to the thermal printhead. Head build-up can become so pronounced, particularly when a lubricant, e.g. polytetrafluoroethylene, is present in the topcoat, that it appears as "spiderwebs" on the thermal printer. "Streaking" is believed to be the result of the insulating effect of head build-up on the printing element(s) of the thermal printhead which interferes with printing causing linear discoloration ("streaking") in the printed image. The presence of a lubricant in the topcoat is generally desired to impart slip characteristics and to decrease gouging of the printed image, however, head build-up usually becomes more pronounced when a lubricant, e.g. polytetrafluoroethylene, is used in the topcoat. Generally, the greater the concentration of lubricant, the greater the degree of head build-up. The aforementioned U.S. Patent No. 5,198,406 of J.M. Mack et al., discloses the use of organofunctional silanes in the topcoat or in a layer on top of the topcoat to react with both the silica and the binder(s) in the topcoat thereby functioning as a coupling agent to join the two and thereby reinforce and strengthen the silica/polymeric binder matrix. The addition of the organofunctional silane helps to reduce head build-up and improves the scratch resistance of the recorded image. SUMMARY OP THE INVENTION The thermographic recording film of the present invention includes an image-forming system and a protective layer comprising colloidal silica, preferably together with a binder material. The film also includes a multiepoxy compound, i.e., a compound containing at least two epoxide moieties, in the protective layer and/or in a layer on top of the protective layer. The multiepoxy compound strengthens and reinforces the thermographic recording film and thereby reduces gouging and head build-up, enhances print performance by decreasing density degradation and improves image quality by decreasing streaking. In a preferred embodiment, the protective layer comprises at least two different colloidal silicas having different average particle size diameters. It is, therefore, among the objects of the present invention to provide thermographic recording materials. DETAILED DESCRIPTION OP THE INVENTION The thermographic recording films according to this invention comprise a support carrying: (a) an image-forming system; and, (b) a protective layer comprising colloidal silica. The thermographic recording film additionally includes a multiepoxy compound in the protective layer and/or in a layer on top of said protective layer. The -1- ratio (by weight) of colloidal silica to said multiepoxy compound is at least 2:1, and preferably in the range of from 2:1 to 15:1; a particularly preferred range is from 2.5:1 to 5:1. At ratios of less than 2:1 there is too little silica present so that sticking may occur. However, at ratios exceeding about 15:1 the integrity of the film tends to be compromised, e.g., crazing and/or cracking of the film may occur. The protective layer of the thermographic recording film may be arranged at different locations within the film dependent upon which surface of the film comes into contact with the thermal printhead during the imaging process. In the embodiment where a layer which is part of the image-forming system contacts the thermal printhead, the protective layer is positioned above the layer(s) comprising the image-forming system. In another embodiment where the support contacts the thermal printhead, such as in a dye diffusion thermal transfer system, the protective layer is arranged on the side of the support which is adjacent the thermal printhead during imaging. The protective layer preferably also includes a binder material, in which case the weight ratio of colloidal silica to the total amount of the multiepoxy compound and binder material combined is at least 2:1 and preferably in the range of from 2:1 to 15:1; a particularly preferred range is from 2.5:1 to 5:1. The absence of a binder in the protective layer generally results in higher levels of haze. Accordingly, the presence of a binder is particularly preferred in the embodiments of the invention where transparency of the imaged film is a concern such as in overhead transparency applications. The transparent supports that can be used in the present invention may be comprised of various materials and numerous suitable support substrates are known in the art and are commercially available. Examples of materials suitable for use as support substrates include polyesters, polycarbonates, polystyrenes, polyolefins, cellulose esters, polysulfones and polyimides. Specific examples include polypropylene, cellulose acetate, and most preferably, polyethylene terephthalate. The thickness of the support substrate is not particularly restricted, but should generally be in the range of about 2 to 10 mils. The support substrate may be pretreated to enhance adhesion of the polymeric coating thereto. The thermographic recording films of the present invention may employ a reflective support in place of the transparent support. Typical suitable reflective supports include polyethylene clad paper such as that sold by Glory Mill Papers Limited (type 381) , Glory Paper Mill, ooburn Green, ylo be, Buchingham Shire, England HPIO ODB; and Baryta coated paper such as that sold by Schoeller Technical Papers Inc. (type 527, Pulaski, New York 13142-0250. Any image-forming system which is suitable for use in thermographic recording films may be utilized in the recording element of the present invention including dye image-forming systems, dye transfer systems and systems where an image material, e.g., a metal complex, is formed as a result of a chemical reaction between two or more system components. A number of suitable image- forming systems are known in the art. Typical suitable image-forming systems which may be incorporated in the recording element of the invention include: A dye image-forming system wherein color- forming di- and triarylmethane dye precursors possessing certain S-containing ring closing moieties, namely a thiolactone, dithiolactone or thioether ring closing moiety, undergo coloration by contact with a Lewis acid material, preferably a metal ion of a heavy metal, particularly silver, capable of opening the S-containing ring moiety to form a colored dye metal complex. A dye image-forming system which utilizes a class of N-substituted triarylmethane sulfonamides which undergo reversible oxidation into the colored form and reversible reduction of the oxidized form into a colorless form as disclosed in U.S. Patent 5 7 258,279. A dye image-forming system wherein a colorless or light-colored basic dye such as a phthalide derivative and a color developer, such as a phenol derivative, capable of causing color development upon contact with the dye are brought together in the presence of an aromatic secondary amine compound as described in U.S. Patent 5,242,884. A dye image-forming system wherein a microencapsulated colorless or light-colored electron donating dye precursor is used in combination with a color developer dissolved in an organic solvent as described in UK patent application GB 2 210 702 A. A system which exploits redox reactions or metal complex formation reactions based on electron donor-acceptor combinations wherein at an increased temperature one of the components melts or diffuses and initiates a redox reaction to provide a colored species; typical of these systems are combinations of: (1) ferric stearate and pyrogallic acid and (2) silver behenate and a suitable reducing agent such as a phenolic compound. Various redox reactions are disclosed in Unconventional Imaging Processes, Focal Press Limited, 1978, page 128. A dye diffusion thermal transfer system wherein a donor layer including a preformed image dye is arranged in combination with an image-receiving layer and an imagewise pattern of the dye is transferred to the image-receiving layer with heat and pressure. As mentioned previously, in this embodiment the protective layer is positioned on the side of the support for the donor layer which is adjacent the thermal printhead during image processing. A system wherein a superacid is liberated from a superacid precursor and takes part in a reaction to provide a colored species as described in copending, commonly-assigned application serial no. 965,161 filed October 23, 1992. It will be understood that various of these systems can be practiced by separating the reactive components from one another such as by placing them in different layers of the element and subsequently causing a desired amount of one reactive component from one layer to diffuse to another layer, as a function of the amount of heat applied, to react with a second component to provide the desired image. A particularly preferred image-forming system for use in the image recording element of the invention is that utilizing di- and triarylmethane thiolactone dye precursors as described in the aforementioned European Patent No. 250,558 and U.S. Patent No. 5,196,297. The dye precursors may be represented by the formula wherein ring B represents a substituted or unsubstituted carbocyclic aryl ring or rings, e.g., of the benzene or naphthalene series or a heterocyclic ring, e.g., pyridine or pyrimidine; G is hydrogen or a monovalent radical; and Z and Z " taken individually represent the moieties to complete the auxochromophoric system of a diarylmethane or a triarylmethane dye when said S- containing ring is open and Z and Z • taken together represent the bridged moieties to complete the auxochromophoric system of a bridged triarylmethane dye when said S-containing ring is open, i.e., when the ring sulfur atom is not bonded to the meso carbon atom. Usually, at least one of Z and Z 1 whether taken individually or together possesses as an auxochromic substituent, a nitrogen, oxygen or sulfur atom or a group of atoms containing nitrogen, oxygen or sulfur. In a preferred embodiment, B is a benzene ring and Z and Z • taken individually or together complete the auxochromophoric system of a triarylmethane dye. The dye precursor compounds used in this embodiment of the invention can be monomeric or polymeric compounds. Suitable polymeric compounds are those which, for example, comprise a polymeric backbone chain having dye precursor moieties attached directly thereto or through pendant linking groups. Polymeric compounds of the invention can be provided by attachment of the dye precursor moiety to the polymeric chain via the Z and/or Z' moieties or the ring B. For example, a monomeric dye precursor compound having a reactable substituent group, such as an hydroxyl or amino group, can be conveniently reacted with a monoethylenically unsaturated, polymerizable compound having a functional and derivatizable moiety, to provide a polymerizable monomer having a pendant dye precursor moiety. Suitable monoethylenically unsaturated compounds for this purpose include acrylyl chloride, methacrylyl chloride, methacrylic anhydride, 2-isocyanatoethyl methacrylate and 2-hydroxyethy1 acrylate, which can be reacted with an appropriately substituted dye precursor compound for production of a polymerizable monomer which in turn can be polymerized in known manner to provide a polymer having the dye precursor compound pendant from the backbone chain thereof. The thiolactone dye precursors can be synthesized, for example, from the corresponding lactones by heating substantially equimolar amounts of the lactone and phosphorus pentasulfide or its equivalent in a suitable solvent. The silver behenate may be prepared in a conventional manner using any of various procedures well known in the art. The polymeric binder for use in this dye- imaging forming system may be any of those binders described in the aforementioned European Patent No. 250,558 and the aforementioned U.S. Patent No. 5,196,297. The preferred polymeric binder is polyvinylbutyral. The organic silver salts which can be employed in this color-forming system of the present invention include any of those described in the aforementioned European Patent No. 250,558 and U.S. Patent No. 5,196,297. Preferred silver salts are the silver salts of long chain aliphatic carboxylic acids, particularly silver behenate which may be used in admixture with other organic silver salts if desired. Also, behenic acid may be used in combination with the silver behenate. The preparation of such organic silver salts is generally carried out by processes which comprise mixing a silver salt forming organic compound dispersed or dissolved in a suitable liquid with an aqueous solution of a silver salt such as silver nitrate or a silver complex salt. Various procedures for preparing the organic silver salts are described in U.S. Patents Nos. 3,458,544, 4,028,129 and 4,273,723. The heat-fusible organic acidic material which can be employed in this embodiment of the invention is usually a phenol or an organic carboxylic acid, particularly a hydroxy-substituted aromatic carboxylic acid, and is preferably 3,5-dihydroxybenzoic acid. A single heat-fusible organic acid can be employed or a combination of two or more may be used. As previously described, the protective layer may include one or more colloidal silicas. The average diameter of the colloidal silicas which may be incorporated in the thermographic recording films of the invention can be up to about 100 nm. It is preferred to utilize colloidal silicas having an average diameter between about 5 nm and about 50 nm. Particularly preferred colloidal silicas are those which have an average diameter of from about 5 nm to about 20 nm. The use of colloidal silicas having an average diameter above 50 nm can result in thermographic recording films which have relatively higher levels of haze and thus which are not as transparent as would be the case when colloidal silicas with smaller average diameters are used. For overhead transparency (OHT) applications, it is desired that the thermographic recording films have a measured level of haze less than 10%, and preferably less than 5%. Thus, for films intended for such applications, it is preferred to utilize colloidal silicas having an average diameter of 50 nm or less. For other applications where haze is of less concern, for example, in reflective thermographic recording films or where the thermal recording film is imaged and subsequently used as a photomask to expose another material, e.g. in the production of circuit boards or diazo prints, etc., a higher level of haze may be tolerated. It should also be noted here that the haze level may be reduced to some extent where a binder is present by choosing a binder which has an index of refraction substantially the same as that of the colloidal silica particles, thus reducing light scatter and resulting haze. One of the colloidal silicas employed in the protective layer of the present invention may be a fumed colloidal silica. Fumed colloidal silica is branched, three-dimensional, chain-like agglomerates of silicon dioxide. The agglomerates are composed of many primary particles which have fused together. Fumed silica is produced by the hydrolysis of silicon tetrachloride vapor in a flame of hydrogen and oxygen. The fumed colloidal silica is referred to as "fumed "1 silica because of its smoke-like appearance as it is formed. If fumed colloidal silica is employed, an average particle diameter in the range of 14-30 nm is generally used, preferably 14-15 nm. When one colloidal silica is used in the protective layer, cracking of the film may be encountered. Accordingly, in high clarity (transparency) applications, it is preferred to include a binder material in the layer and to select the amount of binder so as to overcome any tendency of the film to suffer cracking. A particularly preferred protective layer composition comprises polyvinylalcohol, a diepoxide compound and 5 nm colloidal silica. Such layers exhibit very low haze levels and no, or substantially no, cracking. In a preferred embodiment of the invention, the protective layer comprises a mixture of at least two colloidal silicas having different average particle diameters in the proportion, by weight, of 1 part of silica having an average diameter of 50 nm or less, and about 0.3 to 2 parts of silica particles having an average diameter no more than about 40% of the larger sized colloidal silica particles. The use of two different colloidal silicas helps to prevent cracking in the film. In this embodiment, it is preferred that the largest colloidal silica particles be at least 20 nm in diameter unless fumed colloidal silica is used as the largest sized silica, in which case it is preferred that the fumed colloidal silica be at least 14 nm in diameter. When fumed colloidal silica is employed as the largest sized colloidal silica, it is preferred that the colloidal silicas be present in the proportion, by weight, of 1 part of fumed colloidal silica and 1 to 2.0 parts of silica particles having an average diameter no more than 40% of the larger sized fumed colloidal silica particles. If fumed colloidal silica is not used, it is preferred that the mixture of silicas have different average particle diameters in the proportion, by weight, of 1 part of silica having an average diameter of 50 nm or smaller and 0.3 to 1 part of silica particles having an average diameter no more than 40% of the larger sized silica particles. The mixture of silicas can be utilized to give the hardness and durability necessary to prevent sticking of thermoplastic binder material such as polyvinylbutyral to the thermal printhead, to inhibit scratching on the surface of the thermographic recording film and to limit crazing, i.e., cracking on the surface of the film. The colloidal silicas used in the present invention are produced commercially and typically are provided as an aqueous colloidal dispersion of silica particles in the form of tiny spheres of a specified average diameter. Preferably, the colloidal silicas are aqueous alkaline dispersions, e.g., ammonia stabilized colloidal silica. The fumed colloidal silicas used in the present invention are aqueous dispersions of fumed colloidal silica commercially available under the name Cab-O-Sperse ® from Cabot Corporation, Cab-O-Sil Division, Tuscola, IL. Colloidal silicas and fumed colloidal silicas low in sodium content are preferred since sodium can cause corrosion of the thermal printhead. The binders which can be used in the protective layer of the present invention include both water-soluble and water-insoluble binders. Poor adhesion between the protective layer and color-forming layers with water-soluble binder material has been a problem when a water-soluble binder is used in the absence of the compound containing at least two epoxide moieties. A single binder or a combination of one or more binders can be employed in the protective layer. Examples of water-insoluble binders for use in the protective layer of the present invention include aliphatic polyurethanes, styrene-maleic anhydride copolymers, polyacrylic acid, polyacrylic latex emulsions, polyvinylidene chloride copolymer emulsions and styrene-butadiene copolymer emulsions. Examples of water-soluble binders suitable for use in the protective layer include polyvinylalcohol, polyacrylamide, hydroxyethyl- cellulose, gelatin and starch. To prevent interaction of the components in the protective layer with those in the solvent soluble color-forming layer beneath it, and to ameliorate the environmental concerns associated with coating from solvents, the protective layer of this invention is preferably coated out of aqueous systems. If the binders employed are water-insoluble, they are either coated as latex emulsions or they are made water soluble by mixing with alkali, preferably aqueous ammonia which is lost upon drying. The coating amount of the protective layer is in the range of about 100 to 400 mg/ft 2 . The protective layer preferably contains at least one lubricant, e.g. a wax, a polymeric fluorocarbon such as polytetrafluoroethylene or a metal soap. The preferred lubricant is a polymeric fluorocarbon, e.g. polytetrafluoroethylene. The presence of a lubricant imparts slip characteristics to the thermographic recording film and helps to reduce gouging of the recording film. The protective layer may contain other additives provided the additives do not hinder the anti- stick function of the protective layer, do not damage the thermal printhead or other wise impair image quality. Such additives include surfactants, preferably nonionic surfactants and more preferably nonionic fluorosurfactants; plasticizers; anti-static agents; and ultraviolet absorbers. The multiepoxy compound may be any compound containing at least two epoxide groups provided that the multiepoxy compound is water soluble or water dispersible. Multiepoxy compounds found to be particularly useful in the present invention are diepoxy crosslinking compounds. Examples of suitable diepoxy crosslinking compounds include cycloaliphatic epoxides, e.g. , 3,4-epoxycyclohexylmethyl-3,4- epoxycyclohexanecarboxylate, vinyl cyclohexene dioxide, 2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexane- metadioxane and bis(3,4-epoxycyclohexyl)adipate; 1,4- butanediol diglycidyl ether; 1,2,5,6-diepoxycyclooctane; and 1,2,7,8-diepoxyoctane. When present in the protective layer or in a separate layer on top of the protective layer of the recording films of the present invention, the multiepoxy compounds may be crosslinking with the binder and/or the silica and/or they may be reacting with themselves. The multiepoxy compound may be present in the protective layer itself or in a separate layer on top of the protective layer or it may be present in both the protective layer and in a separate layer on top of the protective layer. Where a multiepoxy compound is present in both the protective layer and a separate layer on top of the protective layer, two different multiepoxy compounds may be used, however, it is preferred that the same multiepoxy compound be used in both layers. The presence of the multiepoxy compound in either layer results in a stronger, more robust protective layer without any substantial impact on the level of haze. The strengthened protective layer results in decreased gouging and enhanced reduction of head build-up. The reduction in head build-up is particularly advantageous when a lubricant is employed in the protective layer. The presence of a lubricant, while often desirable to impart slip characteristics and to decrease gouging, generally increases head build-up. As mentioned earlier, head build-up can cause streaking in the printed image, density degradation over time with continued printing and damage to the thermal printhead. In addition to the above, the presence of the multiepoxy compound provides for both a water and fingerprint resistant film surface. When the multiepoxy compound is present in both the protective layer and in a layer on top of the protective layer, there is generally a more pronounced reduction in head build-up than when the multiepoxy compound is present in only one layer. When the multiepoxy compound is added in the protective layer, the amount employed is calculated to yield, after drying, a coated coverage in the range of 2-40 mg/ft 2 , and preferably 5-15 mg/ft 2 . Where the multiepoxy compound is added in a separate layer on top of the protective layer, it is added as an aqueous solution or an aqueous dispersion and the amount of multiepoxy compound employed is calculated to yield, after drying, a coated coverage in the range of 5-20 mg/ft 2 , preferably 10 mg/ft 2 . Generally, a surfactant is added to the aqueous solution or dispersion of the multiepoxy compound to be coated over the protective layer. The amount of surfactant used is added in an amount calculated to yield, after drying, a coated coverage of 2-5 mg/ft 2 . It has been found that in some instances increased haze levels may be encountered when the coating fluid, containing the multiepoxy compound, for the protective layer is allowed to stand for some period of time, e.g., a few hours, prior to coating the layer. Accordingly, it is preferred to add the multiepoxy compound to the coating dispersion just prior to coating the layer. A preferred protective layer of the present invention comprises a mixture of two different sized colloidal silica particles wherein the largest sized colloidal silica is a fumed colloidal silica having an average particle diameter in the range of 14-30 nm, preferably 14-15 nm and the smaller sized colloidal silica has an average particle diameter of 4 or 5 nm, a diepoxy crosslinking compound added in an amount calculated to yield, after drying, a coated coverage of 15-35 mg/ft 2 , a lubricant, preferably polytetrafluorethylene, and a water-insoluble binder. Fumed colloidal silica has been found to be particularly preferred in thermographic recording films which are imaged with high energy thermal printers such as Model TDU 850 commercially available from Raytheon Company, Submarine Signal Division, Portsmouth, Rhode Island and Model BX 500 commercially available from Seikosha America, Inc., Mahwah, New Jersey. The present invention is illustrated by the following specific examples. Examples 1-16 represent recording elements prepared by coating various protective layer formulations according to the present invention over the identical imaging system. Examples 17 and 18 represent comparative protective layer formulations, which do not contain a multiepoxy compound in or on the protective layer, coated over the same imaging system employed in Examples 1-16. The imaging system employed in each of the examples was prepared by coating Layer One onto a transparent 2.65 mil polyethylene terephthalate substrate pretreated with a solvent adherable subcoat (ICI 505, commercially available from ICI Americas, Inc., Wilmington, DE) by the slot method, followed by air drying. Layer Two was then coated on top of Layer One in the same manner and air dried. It will be appreciated that while slot coating was employed, any appropriate coating method could be used, e.g. spray, air knife, gravure, silkscreen or reverse roll. Both Layer One and Layer Two were coated from a solvent mixture comprised of 80% of methyl ethyl ketone and 20% of methyl propyl ketone. The amounts of components used in each of the layers were calculated to give, after drying, the indicated coated coverages. Layer One: Coverage (mg/ft 2 ) Polyvinylbutyral 386 (Butvar B-72, available from Monsanto, St. Louis, Mo.) 3,5-Dihydroxybenzoic acid 80 Layer Two; Coverage (mg/ft 2 ) Polyvinylbutyral 475 (Butvar B-76, available from Monsanto, St. Louis, Mo.) *Silver behenate dispersion 156 (as silver behenate) Blue Dye Precursor Red Dye Precursor 2 Black Dye Precursor 50 Blue Dye Precursor Red Dye Precursor Black Dye Precursor *The silver behenate dispersion was prepared according to the procedure described on page 29 of the aforementioned European Patent No. 250,558 of E.J. Dombrowski, Jr. et al. Each of the following Examples describes a protective layer formulation which was prepared and coated, either as an aqueous dispersion or as an aqueous solution, over the above described imaging system. The amounts of components used in each protective layer formulation were calculated to give the indicated coated coverages. Example 1 Coverage (mg/ft 2 ) NeoRe.z R966 Polyurethane Latex 25.0 (33% total solids (TS) , available from ICI Resins, Wilmington, MA) Cab-O-Sperse A205 80.0 (a fumed colloidal silica having an average particle diameter of 14 nm, available from Cabot Corporation, Cab-O-Sil Division, Tuscola, IL) Nalco 2326, 5 nm Silica dispersion 80.0 (17% TS, available from Nalco Chemical Co.) Hostaflon 5032, polytetra- 0.5 fluoroethylene dispersion, (60% TS, available from Hoechst-Celanese, Chatham, NJ) Zonyl FSN, perfluoroalkyl polyethylene 5.0 oxide non-ionic surfactant available from DuPont, Wilmington, DE) 1,4-Butanediol diglycidyl ether 20.0 (commercially available as Araldite DY 026 SP from Ciba-Geigy Limited -(Plastics Division) . 05 Example 2 Coverage (mg/ft 2 ) NeoRez R966 Polyurethane Latex 35.0 Cab-O-Sperse A205, fumed colloidal 65.0 silica Nalco 2326, 5 nm Silica dispersion 90.0 Hostaflon 5032, polytetra¬ 0.5 fluoroethylene dispersion Zonyl FSN 5.0 1,4-Butanediol diglycidyl ether 25.0 Example 3 Coverage (mg/ft 2 ) NeoRez R966 Polyurethane Latex 38.4 Cab-O-Sperse A205, fumed colloidal 71.3 silica Nalco 2326, 5 nm Silica dispersion 98.7 Hostaflon 5032, polytetra¬ 5.5 fluoroethylene dispersion Zonyl FSN 5.5 1, 4-Butanediol diglycidyl ether 27.4 Example 4 Coverage (mg/ft 2 ) NeoRez R966 Polyurethane Latex 25.0 Cab-O-Sperse A205, fumed colloidal 80.0 silica Nalco 2326, 5 nm Silica dispersion 80.0 Zonyl FSN 5.0 1, 4-Butanediol diglycidyl ether 20.0 Example 5 A recording element was prepared according to example 4, above, and was subsequently coated with an aqueous mixture of 1,4-butanediol diglycidyl ether and Zonyl FSN. The amounts of each component used were calculated to give the indicated coated coverages after drying at 145"F (~63 β C) for 3 minutes: Coverage (mg/ft 2 ) 1,4-Butanediol diglycidyl ether 10 Zonyl FSN 3 Example 6 Coverage (mg/ft 2 ) NeoRez R966 Polyurethane Latex 25.0 Cab-O-Sperse A205, fumed colloidal 80.0 silica Nalco 2326, 5 nm Silica dispersion 80.0 Zonyl FSN 5.0 The above prepared recording element was subsequently coated with an aqueous mixture of 1,4- butanediol diglycidyl ether and Zonyl FSN as described in Example 5. Example 7 Coverage (mg/ft 2 ) NeoRez R966 Polyurethane Latex 25.0 Cab-O-Sperse A205, fumed colloidal 65.0 silica Nalco 2326, 5 nm Silica dispersion 90.0 Hostaflon 5032, polytetra¬ 0.5 fluoroethylene dispersion Zonyl FSN 5.0 1,4-Butanediol diglycidyl ether 10.0 Example 8 Coverage (mg/ft 2 ) NeoRez R966 Polyurethane Latex 30.0 Cab-O-Sperse A205, fumed colloidal 96.0 silica Nalco 2326, 5 nm Silica dispersion 96.0 Zonyl FSN 6.0 1,4-Butanediol diglycidyl ether 24.0 The above prepared recording element was subsequently coated with an aqueous mixture of 1,4- butanediol diglycidyl ether and Zonyl FSN as described in Example 5. Example 9 Coverage (mg/ft 2 ) Polyvinyl alcohol, Vinol 350 25.0 (available from Monsanto, St. Louis, Mo.) Cab-O-Sperse A205, fumed colloidal 65.0 silica Nalco 2326, 5 nm Silica dispersion 90 . 0 Zonyl FSN 5 . 0 l,4-Butanediol diglycidyl ether 20 . 0 Example 10 Coverage (mg/ft 2 ) NeoRez R966 Polyurethane Latex 35 . 0 Cab-O-Sperse A205, fumed colloidal 65 . 0 silica I Nalco 2326, 5 nm Silica dispersion 90 . 0 Hostaflon 5032, polytetra¬ 0 . 5 fluoroethylene dispersion Zonyl FSN 5 . 0 Bis(3,4-epoxycyclohexyl)adipate 25 . 0 (commercially available from Union Carbide Corp., Danbury, CT) Example 11 A recording element was prepared according to example 9, above, and was subsequently coated with an aqueous mixture of 1,4-butanediol diglycidyl ether and Zonyl FSN as described in Example 5. 905 Example 12 Coverage (mg/ft 2 ) NeoRez R966 Polyurethane Latex 25.0 Cab-O-Sperse A205, fumed colloidal 65, 0 silica Nalco 2326, 5 nm Silica dispersion 90. Hostaflon 5032, polytetra¬ 1. fluoroethylene dispersion Zonyl FSN 5.0 1, 4-Butanediol diglycidyl ether 10.0 Example 13 A recording element was prepared according to example 11, above, and was subsequently coated with an aqueous mixture of 1,4-butanediol diglycidyl ether and Zonyl FSN as described in Example 5. Example 14 Coverage (mg/ft ) Cab-O-Sperse A205, fumed colloidal 80.0 silica Nalco 2326, 5 nm Silica dispersion 80.0 Hostaflon 5032, polytetra¬ 0.5 fluoroethylene dispersion Zonyl FSN 5.0 1, 4-Butanediol diglycidyl ether 20.0 Example 15 Coverage (mg/ft 2 ) NeoRez R966 Polyurethane Latex 25.0 Cab-O-Sperse A205, fumed colloidal 65.0 silica Nalco 2326, 5 nm Silica dispersion 90.0 Hostaflon 5032, polytetra- 0.5 fluoroethylene dispersion Zonyl FSN 5.0 The above prepared recording element was subsequently coated with an aqueous mixture of 1,4- butanediol diglycidyl ether and Zonyl FSN as described in Example 5. Example 16 Coverage (mg/ft 2 ) Nalco 2326, 5 nm Silica dispersion 180.0 Vinol 540 (polyvinylalcohol 25.0 (available from Monsanto, St. Louis, MO) Zonyl FSN 5.0 1,4-Butanediol diglycidyl ether 20.0 Comparative Example 17 Coverage (mg/ft 2 ) NeoRez R966 Polyurethane Latex 25.0 Cab-O-Sperse A205, fumed colloidal 65.0 silica Nalco 2326, 5 nm Silica dispersion 90.0 Hostaflon 5032, polytetra¬ 0.5 fluoroethylene dispersion Zonyl FSN 5.0 Comparative Example 18 Coverage (mg/ft 2 ) NeoRez R966 Polyurethane Latex 25.0 Cab-O-Sperse A205, fumed colloidal 80.0 silica Nalco 2326, 5 nm Silica dispersion 80.0 Zonyl FSN 5.0 Each of the recording elements prepared above, except for the one prepared in Example 3, were imaged by means of a Model TDU 850 direct thermal printer, commercially available from Raytheon Company, Submarine Signal Division, Portsmouth, Rhode Island. Example 3 was imaged with a Model BX 500 direct thermal printer, commercially available from Sei osha America, Inc., Mahwah, NJ. When using a Model BX 500 printer to image, the thermographic recording media of the present invention preferably include a lubricant in the topcoat in amount to give a coated coverage after drying of 4.0 to 6.0 mg/ft 2 . When using other high energy printers, e.g., the Model TDU 850, a lesser amount of lubricant, i.e. 0.25 to 1.0 mg/ft 2 , is generally employed. The streaking, % haze, the amount of gouging and the head build-up were determined for each imaged film. The results are recorded in Table 1. The haze measurements were determined using a Spectrogard II Spectrophotometer made by Gardner-Neotec Instruments, Silver Spring, Maryland. Streaking, gouging and head build-up were each ascertained visually. For streaking, "excellent" describes those recording films for which there was no observable streaking after 50 feet of printing; "very good" describes those recording films for which there was only slight, but noticeable streaking after 50 feet of printing; "good" describes recording films for which there was moderate streaking visible after 50 feet of printing; "fair" is used to describe those recording films for which there was heavy streaking before 50 feet of printing accompanied by significant density loss; and, "poor" describes those recording films for which streaking was so severe that 50 feet of recording film could not be successfully printed - the heating elements were insulated to an extent which seriously interfered with printing. For gouging, "excellent" describes those recording films for which there was no observable gouging after 50 feet of printing; "fair" describes those recording films for which infrequent gouging was observed in the high density areas of the images; and, "poor" describes those recording films for which severe gouging was observable at the onset of printing. For head build-up, "excellent" describes those situations in which there was only very slight if any head build-up on the thermal printhead after 50 feet of printing; "good" describes those situations where there was a slight to moderate accumulation of material on and/or after the print elements after 50 feet of printing; "fair" describes those situations for which there was substantial accumulation of material on and/or after the print elements after 50 feet of printing; and, "poor" describes those situations in which there was an exorbitant amount of material directly on and after the print elements. TABLE I The level of haze in examples 9 and 14 is noted as being relatively higher than that reported for the other examples. The high level of haze in example 9 is believed to be due to crosslinked polyvinylalcohol coming out of solution during the drying process when the film was formed. The high level of haze in example 14 is attributed to the absence of binder in the topcoat. As can be seen from the results shown in Table 1, the thermographic recording films of Examples 1-16 according to the present invention were superior in terms of gouging (for those recording films which did not contain any lubricant) , head build-up, and streaking to comparative Examples 17-18 which did not contain a diepoxy crosslinking compound in the protective layer and/or in a layer on top of the protective layer. To further illustrate the present invention, recording films prepared as in Examples 2, 4, 5, 6 and 16 were continuously imaged with a test pattern having an eight-step gray tone scale. Measurements of the optical transmission density (O.D.) of each of the gray steps were made. Tables 2-6 show the initial density of each of the gray steps, the density of the gray steps after imaging 50 feet of recording film and the difference between the two measurements (O.D. Δ) for each of examples 2, 4, 5, 6 and 16 respectively. The densities reported after 50 feet of printing were obtained after continuously printing for 50 feet, stopping, allowing the printer to cool for 10 minutes, restarting the printing and measuring the resulting transmission density. This was done to compensate for any density loss attributable to the thermal printer. The built-in electronics of the thermal printhead do not sufficiently compensate for heat build-up in the head itself and consequently some density loss tends to occur upon continued printing, independent of the particular thermographic recording film. As a control, the experiment was repeated using a recording film prepared according to comparative example 17; the results are reported in Table 7. TABLE 2 Example 2 Step Initial O.D. O.D. 50 ft O.D. Δ 1 0.28 0.29 -0.01 2 0.35 0.35 0.00 3 0.42 0.44 -0.02 4 0.48 0.46 0.02 5 0.54 0.55 -0.01 6 0.71 0.69 0.02 7 0.92 0.95 -0.03 8 1.76 1.79 -0.03 TABLE 3 Example 4 Step Initial O.D. O.D. 50 ft O.D. Δ 1 0.33 0.32 0.01 2 0.40 0.42 -0.02 3 0.50 0.50 0.00 4 0.57 0.56 0.01 5 0.65 0.66 -0.01 6 0.78 0.78 0.00 7 1.01 1.01 0.00 8 1.84 1.85 -0.01 TABLE 4 Example 5 Step Initial O.D. O.D. 50 ft O.D. Δ 1 0.32 0.32 0.00 2 0.40 0.41 -0.01 3 0.49 0.48 0.01 4 0.56 0.54 0.02 5 0.66 0.65 0.01 6 0.80 0.79 0.01 7 1.03 1.00 0.03 8 1.83 1.81 0.02 TABLE 5 Example 6 Step Initial O.D. O.D. 50 ft O.D. Δ 1 0.29 0.19 0.10 2 0.35 0.26 0.09 3 0.46 0.35 0.11 4 0.50 0.39 0.11 5 0.64 0.55 0.09 6 0.74 0.68 0.06 7 0.99 0.92 0.07 8 1.84 1.79 0.05 TABLE 6 Example 16 Step Initial O.D. O.D. 50 ft O.D. Δ 1 0.24 0.27 0.03 2 0.32 0.27 -0.05 3 0.48 0.46 -0.02 4 0.54 0.57 0.03 5 0.66 0.72 0.06 6 0.79 0.84 0.05 7 1 1.19 0.19 8 1.61 1.76 0.15 TABLE 7 Comparative Example 17 Step Initial O.D. O.D. 50 ft O.D. Δ 1 0.14 0.05 0.09 2 0.20 0.10 0.10 3 0.27 0.12 0.15 4 0.31 0.14 0.17 5 0.44 0.20 0.24 6 0.57 0.39 0.18 7 0.78 0.55 0.23 8 1.44 1.28 0.16 As can be seen from the foregoing data, the recording films of the present invention which contain a multiepoxy compound in the protective layer and/or in a layer on top of the protective layer, decrease the density degradation which may occur over time with continued printing. It is noted that Example 6, which had only 10 mg/ft 2 of 1,4-butanediol diglycidyl ether in the protective layer, showed some density degradation with continued printing. However, the density loss was less than that observed in comparative example 17, which contained no multiepoxy compound in the protective layer. Since certain changes may be made in the above subject matter without departing from the spirit and scope of the invention herein involved, it is intended that all matter contained in the above description and the accompanying examples be interpreted as illustrative and not in any limiting sense.

Claims: We claim: 1. A thermographic recording film comprising a support carrying: (a) an image-forming system; and, (b) a protective layer comprising at least one colloidal silica, said thermographic recording film additionally including a compound containing at least two epoxide moieties in the protective topcoat layer and/or in a layer on top of said protective topcoat layer, the ratio of colloidal silica to said compound containing at least two epoxide moieties being at least 2:1 by weight. 2. A thermographic recording film according to claim 1 wherein said protective layer additionally includes a binder. 3. A thermographic recording film according to claim 2 wherein said binder in said protective layer is a water-insoluble binder. 4. A thermographic recording film according to claim 3 wherein said water-insoluble binder is an aliphatic polyurethane. 5. A thermographic recording film according to claim 2 wherein said binder in said protective layer is a water-soluble binder. 6. A thermographic recording film according to claim 5 wherein said water-soluble binder is polyvinylalcohol. 7. A thermographic recording film according to claim 1 wherein said compound containing at least two epoxide moieties is a diepoxy crosslinking compound. 8. A thermographic recording film according to claim 7 wherein said diepoxy crosslinking compound is 1,4-butanediol diglycidyl ether. 9. A thermographic recording film according to claim 7 wherein said diepoxy crosslinking compound is bis(3,4-epoxycyclohexyl)adipate. 10. A thermographic recording film according to claim 1 wherein said compound containing at least two epoxide moieties is present in said protective layer. 11. A thermographic recording film according to claim 10 which additionally includes a compound containing at least two epoxide moieties in a layer on top of said protective layer. 12. A thermographic recording film according to claim 1 wherein said compound containing at least two epoxide moieties is present in a layer on top of said protective layer. 13. A thermographic recording film according to claim 1 wherein said protective layer comprises at least two colloidal silicas having different average particle diameters in the proportion by weight of 1 part of silica particles having an average diameter of 50 nm or smaller, and 0.3 to 2.0 parts of silica particles having an average particle diameter no more than 40% of the larger sized silica particles. 14. A thermographic recording film according to claim 13 wherein one of said colloidal silicas is a fumed colloidal silica having an average particle diameter in the range of 14 nm to 30 nm. 15. A thermographic recording film according to claim 1 wherein said protective layer comprises 2 colloidal silicas, wherein one of said colloidal silicas is fumed colloidal silica having an average diameter of 14 nm. 16. A thermographic recording film according to claim 15 wherein the second colloidal silica is a colloidal silica having an average particle diameter of 5 nm. 17. A thermographic recording film according to claim 1 wherein said image-forming system is a dye image-forming system. 18. A thermographic recording system as defined in claim 17 wherein said dye image-forming system comprises a di- or triarylmethane thiolactone dye precursor, an organic silver salt, a heat-fusible organic acidic material and a polymeric binder. 19. A thermographic recording film according to claim 18 wherein said organic silver salt, said polymeric binder and said thiolactone dye precursor are carried in one layer on said support and said heat- fusible organic acidic material is in an adjacent layer. 20. A thermographic recording film according to claim 18 wherein said dye image-forming system further comprises a second heat-fusible organic acidic material. 21. A thermographic recording film according to claim 1 wherein said protective layer further comprises a lubricating agent. 22. A thermographic recording film according to claim 21 wherein said lubricating agent is polytetrafluoroethylene. 23. A thermographic recording film according to claim 1 wherein the ratio of said silica to said compound containing at least two epoxide moieties is from 4:1 to 8:1. 24. A thermographic recording film according to claim 2 wherein the ratio of said silica to the total of said binder and said compound containing at least two epoxide moieties is 2.5:1 to 5:1. 25. A thermographic recording film according to claim 1 wherein said protective layer further comprises a surfactant. 26. A thermographic recording film according to claim 25 wherein said surfactant is a nonionic fluorosurfactant. 27. A thermographic recording film according to claim 2 wherein said polymeric binder is polyvinylbutyral. 28. A thermographic recording film according to claim 2 wherein said protective layer further comprises a second binder. 29. A thermographic recording film according to claim 1 wherein said support is a transparent support. 30. A thermographic recording film according to claim 1 wherein said protective layer is positioned above said image-forming system. 31. A thermographic recording film according to claim 1 wherein said protective layer is positioned on the surface of said support opposite that which carries said image-forming system.

Inventors: DOMBROWSKI EDWARD J, MCPHERSON JOHN R SR, DOMBROWSKI, EDWARD, J., MCPHERSON, JOHN, R., SR.
Applicants: POLAROID CORP, POLAROID CORPORATION
Application year: 1994
Main classification: B41M5
IPC codes: B41M5, G03C1